In communication systems such as the Integrated Services Digital Network (ISDN) basic access, the digital information is exchanged between terminals in analog form over a two-wire transmission path. Each terminal includes an ISDN basic access transceiver adapted to transmit a digital stream coded in analog form and to convert a received analog signal to a digital stream. While the received signal is analog, the ISDN basic access receiver processing is generally performed digitally to take advantage of the better precision, lower noise and the large scale integration economy of digital processing. It is therefore necessary to convert an incoming analog signal at the receiver to digital form preparatory to the digital processing. The analog-to-digital conversion preceding the digital processing must be very precise to avoid severe distortion. Such precision may be obtained by using large high resolution converters with compensation networks, component trimming or oversampling of signals.
U.S. Pat. No. 4,746,902 issued to Tol et al, May 24, 1988, discloses an arrangement that compensates for nonlinear distortion in an input signal to be digitized having an analog-to-digital converter that converts the input signal into an amplitude-time discrete output signal. Processing apparatus derives a set of coefficients associated with an orthogonal signal representation of a signal related to the input signal. A table storing correction values is addressed by a signal derived from the set of coefficients to produce a correction value. The correction value and the analog-to-digital converter output signal are summed to provide a linearized signal and an adaptive control loop substitutes a new correction value for the correction value read from the table. While the arrangement corrects for distortion generated by the analog-to-digital converter, it has limited application and does not improve the resolution of the converter or reduce its complexity or size.
U.S. Pat. No. 4,612,533 issued to Evans, Sept. 16, 1986, describes a harmonic distortion reduction technique for data acquisition to provide fast conversion rates in which a subrange digital-to-analog converter (DAC) is augmented with a software calibrated DAC to remove system nonlinearities. Harmonic distortion is reduced by adding a correction to the input of the subrange ADC or by using a look up table at the output of the ADC. While the harmonic distortion reduction is achieved, it requires both a main ADC and a subrange DAC converter. The resulting arrangement, however, is highly dependent on analog components and analog processing and is relatively slow since it is necessary to delay the operation of the subrange DAC until each cycle of operation of the main ADC is completed.
In transmission systems where transceivers are employed for full duplex operation, a large component of the received signal is an echo of the signal being transmitted. The echo due to the transmitted signal and other components of the received signal may be synthesized from the information being processed for transmission and other sources. The aforementioned arrangements provide conversion of an unknown analog signal into a high resolution digital form without utilizing such previously known characteristics. It is an object of the invention to provide improved resolution analog-to-digital conversion in communication systems for signals having known or synthesizable components.